1. Field of the Invention
The present invention relates to an optical pickup and an optical information processing apparatus using the optical pickup by which a satisfactory beam spot on an information recording surface of a blue-system optical recording medium which uses a light source of a blue wavelength zone, to an DVD-system optical recording medium using a light source of a red wavelength zone, or to a CD-system optical recording medium using a light source of an infrared wavelength zone.
2. Description of the Related Art
Optical recording media, such as a CD with a storage capacity of 0.65 GB and a DVD with a storage capacity of 4.7 GB, are spreading as means to store data of image information, voice information, or computer data. Further, demands for a further improvement in recording density and storage capacity have become stronger in recent years. Specifically, the necessity for as storage capacity such as for 22 GB by which a high-definition television program can be stored for two hours for recording one movie program, or for 44 GB by which the same can be stored for four hours for recording a sport relay broadcast or so.
In an optical pickup which performs information writing or informational reading to/from an optical recording medium, in order to improve the recording density of such an optical recording medium, it is effective to reduce the diameter of the beam spot which is formed by a beam condensed with an object lens and formed on an optical recording medium by enlarging the numerical aperture (which may be abbreviated as ‘NA’) of the object lens, or by shortening the wavelength of a light emitted by a light source. Therefore, for example, in a “DVD-system optical recording medium” with which high recording density is achievable for which NA of the object lens is set as 0.65 and the light source is set to emit a light of the wavelength of 660 nm in comparison to a “CD-system optical recording medium” for which NA of the object lens is set as 0.50 and the light source is set to emit a light with the wavelength of 780 nm.
As for such an optical recording medium, as mentioned above, further improvement in the recording density and storage capacity is demanded, and, for this purpose, it is desirable to increase from 0.65 in NA of the object lens, or to make the wavelength of the light source shorter than 660 nm. In this regard, in a document of an ISOM 2001 collection of preliminary reports ‘Next Generation Optical Disc’, by Hiroshi Ogawa, pages 6–7, a system is proposed by which the storage capacity of 22 GB is achievable by using a light source of a blue wavelength zone and an object lens of NA: 0.85, for example. Further, another system is proposed in which 12 GB in the storage capacity is achievable by employing a light source of a blue wavelength zone, with NA of 0.65–0.75.
However, when a further improvement in NA of the object lens or the wavelength of the light source is attained, various problems are anticipated. The anticipated problems include, for example, those caused by a manufacture error of a lens, an even-th aberration which means a distortion in an isotropic wavefront with respect to the optical axis produced according to a manufacture error of a lens applied, a thickness error of a transparent substrate of an optical recording medium loaded, an odd-th aberration which means a distortion in an anisotropic wavefront with respect to the optical axis generated by an inclination of the optical recording medium, or so.
Generally, the even-th aberration generated according to a thickness error of a transparent substrate of an optical recording medium is given the following spherical aberration:W40=((n2−1)/(8n3))×(d×NA4/λ)                where n denotes the refractive index of the transparent substrate of the optical recording medium, d denotes the thickness of the same, NA denotes the numerical aperture of the object lens, and, λ denotes the wavelength of the light source.        
From the above formula, it can be seen that, as the wavelength becomes shorter or NA becomes higher, the aberration becomes larger accordingly. In an optical information processing device which performs writing/reading of information onto/from a conventional optical recording medium, such as a CD or a DVD, since the wavefront degradation by this spherical aberration is as small as less than 0.07λ in the RMS value, not a special means of correction of the aberration is needed. However, when the operating wavelength of 407 nm±10 nm in a light source of a blue wavelength zone is used, the wavefront degradation caused by such a thickness error of the transparent substrate becomes more than 0.07λ, and, therefore, some sort of correction becomes required.
For this purpose, various proposals have been made. For example, proposed is a means to change the phase state of an incident beam to an object lens (Japanese laid-open patent applications Nos. 2000-131603, 2000-242963, and 2001-28147), a means to change the divergence state of the beam incident onto the object lens by means of an appropriate movement of a plurality of lenses (Japanese laid-open patent applications Nos. 9-128785, and 10-20263), and so forth.
Moreover, generally the above-mentioned odd-th aberration generated by an inclination, a rotation or so of an optical recording medium is given by the following coma aberration:W31=((n2−1)/(2n3))×(d×NA3×/λ)                where θ denotes an inclination of the optical recording medium.        
From this formula, it can be seen that the shorter wavelength or higher NA results in an increase in this sort of aberration. In an optical information processing device which performs writing/reading of the information onto/from a CD, since the wavefront degradation caused by this coma aberration is as low as 0.07λ in the RMS value, no special means for correcting it is needed. However, when the operating wavelength of 660 nm±10 nm in the light source of a red wavelength zone is used, the wavefront degradation caused by a change of ±1 degree in the inclination results in 0.07λ, and, thus, some sort of correction is required in a generation of DVD-system information recording media. Proposals have been made for such a correction, for example, in Japanese laid-open patent applications Nos. 10-91990, 2001-220075, Japanese patent No. 3142251, Japanese laid-open patent application No. 9-128785, etc.
Moreover, in such an optical pickup, a beam emitted from a light source is focused onto an optical recording medium in one light path, while a light reflected from the optical recording medium is detected by a light detecting device in another light path. To improve the light usage efficiency in both of these light paths is desired. Namely, as a consideration is made into a matter as to whether or not the light usage efficiency in the going light path until the beam emitted from the light source is incident on the optical recording medium, it becomes possible to raise the condensing power for a beam spot on the optical recording medium by which the rotational speed of the optical recording medium can be improved. Alternatively, in case merely the same condensing power for the optical beam is required, it becomes possible to reduce the required power in the light source, and thus cost saving can be realized.
On the other hand, in case the light usage efficiency on the returning light path of being directed toward the light detection device from the optical recording medium can be made higher, the optical signal from the optical recording medium can be detected at a high S/N. Morio Onoe discloses a scheme for improving the light usage efficiency in both the going and returning light paths in a document ‘Optical Technology’, published by Radio Technology Co., Ltd., 1989, pages 66–68, in which a ¼-wavelength plate and a polarization splitter are disposed in the light path for this purpose.
Generally speaking, such a scheme of applying a polarization beam splitter and a phase plate or the like is preferable for a blue-system optical recording medium with a wavelength of 407 nm and a DVD-system optical recording medium with a wavelength of 660 nm. On the other hand, for a CD-system optical recording medium with a wavelength of 780 nm, many optical pickups using a light path separation device which does not perform polarization have been put into commercial use. However, when a light path separation device which does not perform polarization is used, a part of light reflected from the optical recording medium returns to the light source. In case of using a semiconductor laser as the light source, when a part of reflected light from the optical recording medium returns to the semiconductor laser, the oscillation characteristic of the semiconductor laser is changed, and, thus, there occurs a possibility of generating a noise, at a time of information recording/reproduction/deletion onto the optical recording mediums.
Such a matter of noise generation by the returning light does not almost become a serious problem in an optical pickup used only for information reproduction. However, in case of using a high output type semiconductor laser in which the oscillation characteristic is likely to change, and, thus, the above-mentioned noise generation may cause a serious problem in an optical pickup also used for information recording. Japanese laid-open patent application No. 11-261171 discloses an optical pickup directed to solving such a problem. In the disclosed configuration, a wavelength plate is disposed for the purpose of eliminating such a noise occurring due to a returning light in a single-wavelength optical system.
On the other hand, according to the above-mentioned document of ISOM 2001 collection of preliminary reports ‘Next Generation Optical Disc’, by Hiroshi Ogawa, pages 6–7, a system using a light source of a blue wavelength zone in an operating wavelength of 407 nm±10 nm and an object lens of NA of 0.85 is such that a setting is made on the optical-incident side substrate thickness of an optical recording medium as being 0.1 mm, and, thereby, an aberration increase amount occurring due to reduction in wavelength or increase in NA may be compensated, and, thus, even if there occurs a wavefront degradation caused due to an inclination change of the optical recording medium, it may be kept not more than 0.07λ.
On the other hand, a manufacture error in products of optical devices, especially, as an object lens, used in an optical pickup may cause a problem concerning an aberration. Generally, a thickness error, a refractive-index error, a curvature radius error and so forth in the lens may cause generation of the even-th aberration, while a tilt or eccentricity of the lens may cause the odd-th aberration.
Another problem is such that users still have conventional recording medium such as CDs and DVDs. It is desirable that these conventional optical recording media and new optical recording media according to the above-mentioned new standard should be handled with a single common optical information processing device. One easier method of solving this problem is such that an optical pickup according to the conventional standard and an optical pickup according to the new standard are both mounted in one machine separately. However, this method may raise the cost or increase the size of the whole machine.
Thus, problems to be solved occurring when achieving an optical pickup directed to the future of high-density optical recording media are to reduce aberrations which otherwise increase due to increase in the NA or reduction in wavelength, and, also, to achieve a compatibility between the conventional standard and new standard optical recording media (i.e., of different generations). Further, it is also a problem to be solved to achieve these objects without a remarkable increase in size/costs of the machine.
Furthermore, as described above, as long as the conventional recording media such as CDs and DVDs are applied, spherical aberration occurring in connection with a thickness error of optical recording medium may be reduced by means of improvement in the manufacture accuracy of the optical recording medium. Further, coma aberration in connection with an inclination of an optical recording medium may also be reduced by setting the substrate thickness thereof into 0.1 mm, even when the light source of a blue wavelength zone and NA of 0.85 are applied. However, in the future, in connection with achievement of high-speed rotation of an optical recording medium, application of a multi-layer-type optical recording medium, which will be described later, and also application of a multi-level recording scheme, it may become not possible to tolerate such an aberration matter, and thus, some special scheme for correcting it should be needed.